This project has been aimed at identifying and characterizing the T cell subsets that initiate and effect anti-MHC responses in vitro. We have identified and characterized 3 distinct T helper cell populations that function in vitro to initiate anti-MHC responses, and have studied 3 distinct T killer cell populations that function in vitro as anti-MHC effector cells. We have characterized these cell populations with regard to their specificity, phenotype, activation requirements, and differentiation requirements.